1. Technical Field
This invention relates generally to aluminide coatings and particularly to aluminide coatings which are resistant to oxidation degradation and thermal fatigue cracking.
2. Background Information
Aluminide coatings are known to provide oxidation and corrosion protection for superalloy articles, such as blades and vanes, used in gas turbine engines. Such coatings are favored in the gas turbine engine industry because they are economical and add little weight to the engine.
Aluminide coatings may be formed by a pack process wherein a powder mixture, including an inert material, a source of aluminum, and a halide activator is employed. The superalloy to be coated is inserted into a coating box and covered with the powder mixture or pack. The coating box is then placed in a retort. A reducing or inert gas is then flowed through the retort. During the coating process, the halide activator reacts with the source of aluminum and produces an aluminum-halide vapor which circulates over the surface of the superalloy article. Upon contact with the surface of the superalloy article, the vapor decomposes and deposits aluminum on the superalloy surface whereby the halide is released and contacts the aluminum source to continue the chemical reaction. The deposited aluminum then combines with nickel from the superalloy surface thereby forming an aluminum-rich surface layer or coating on the superalloy article. Use of this pack process is advantageous when it is desired to coat the entire surface of a superalloy article. However, it is difficult to coat select portions of the article without the employment of detailed masking techniques.
Another known technique for forming an aluminum-rich surface layer on a superalloy article is a vapor phase aluminiding process. Generally, in this process the superalloy article is suspended in an out-of-contact relationship with the above described powder mixture as opposed to being embedded within the powder mixture. However, problems are associated with some vapor phase aluminiding processes. For example, formation of undesirable oxides within the coating itself and on the original substrate surface may be encountered. These oxides are undesirable because they may degrade the coating properties.
U.S. Pat. No. 3,102,044 to Joseph describes another method of forming an aluminum-rich surface layer on a superalloy article. In this method an aluminum-rich slurry is applied to the superalloy surface and heat treated to form a protective aluminide coating thereon. Although such aluminum-rich slurry techniques can be successful in producing a protective aluminide coating on the surface of the superalloy article, it is very labor intensive and time consuming to coat an entire superalloy article in this fashion. Achieving coating uniformity from one location on the article surface to another can be difficult. Furthermore, even if it is desired to coat only a portion of the article, such as a small area damaged during engine operation or damaged during handling in the manufacturing process, care must be taken in applying the slurry only to those areas in need of coating. Thus, detailed masking techniques may be necessary.
U.S. Pat. No. 5,334,417 to Rafferty et al. describes yet another method of producing an aluminide coating. Specifically, Rafferty et al. disclose a method for forming a pack cementation coating on a metal surface by a coating tape. The tape includes elemental metal, a filler, a halogen carrier composition and a binder material, specifically fibrillated polytetrafluoroethylene. According to Rafferty et al., the components are formed into a malleable tape and cut to the desired size. To form the pack cementation coating, the tape is placed on the surface of the part which is put in an oven and heated to a temperature of about 1250.degree. F. (677.degree. C.) to 1350.degree. F. (732.degree. C.) for 0.5 to about 3 hours with the typical time being about 1.5 hours. The process causes a chemical reaction to occur in which fluoride or chloride compound breaks down to form halide ions which react with the metal (or metal alloy) atoms forming the metal halide compound. When the metal halide contacts the base metal surface, the metal in the metal halide compound is reduced to elemental metal which can alloy with the base metal. More specifically, metal ions, such as aluminum, vanadium or chromium react with the nickel, iron or cobalt of the base metal to form the aluminide or nickel vanadium or nickel chromium composition.
Although Rafferty et al. seem to address the need for an efficient way to coat select portions of gas turbine engine components, the above described resultant coating does not appear to be a fully diffused coating. Thus, it is brittle and may be dislodged from the component, for example, during handling or during engine operation.
Notwithstanding the advances made in the aluminiding field, scientists and engineers under the direction of Applicant's Assignee continue in their attempts to develop aluminide coatings. Such coatings must have excellent resistance to oxidation and corrosion attack and must be particularly resistant to thermal fatigue cracking, as well as economical and easy to apply, particularly to select portions of gas turbine engine components. The invention results from such effort.